TECTONOPHYSICS ELSEVIER

Tectonophysics 274 (1997) 61-82

Quaternary transfer faulting in the Taiwan Foothills: evidence from a multisource approach B. Deffontaines a,*, O. Lacombe a, j. Angelier a, H.T. Chu b, E Mouthereau a, C.T. Lee c, J. Deramond d, J.E Lee b, M.S. Yu b, P.M. Liew e a D~partement de G~otectonique, URA-CNRS 1759, Universit( Pierre et Marie Curie, T. 26, El, Case 129, 4 Place Jussieu, F-75252, Paris, Cedex 05, France b Central Geological Survey, MOEA, PO Box 968, Taipei, Taiwan c National Central University, Institute of Applied Geology, Chungli, Taiwan d Laboratoire de G~ologie Structurale et Tectonophysique, Universit( Paul Sabatier, 38, rue des 36 Ponts, F-31400, Toulouse, France e Geological Department, National Taiwan University, 245 Choushan Road, Taipei, Taiwan Received 12 February 1996; accepted 30 July 1996

Abstract The major structures of the Western Foothills of Taiwan mainly consist of NNE-SSW-trending folds and imbricated west-vergent thrust systems. The additional occurrence of N140°E-striking oblique structures was revealed through a multisource approach involving a Digital Elevation Model (DEM), a study of drainage network anomalies, aerial photographs, Side-Looking Airborne Radar (SLAR) images and SPOT-P and Landsat images. These structures are described from north to south based on new field analyses (including stratigraphy and tectonics studies). They are also compared to seismic data and geodetic reconstruction, in order to evaluate their present-day activity. These NI40°E major morphostructures are interpreted as left-lateral transfer fault zones, either inherited from the Eurasian passive margin and/or newly formed in the cover in response to the presence of basement highs within the foreland basin (Peikang and Kuanyin highs). The Sanyi and the Chishan transfer fault zones display a high seismic activity; the distribution of earthquakes and the related focal mechanisms confirm the left-lateral movement along N140°E directions. The Chiayi, Chishan, and Fengshan fault zones act presently as transfer fault zones, as indicated by GPS data. The associated N70°E- to N100°E-trending faults result from the reactivation of normal faults of the Eurasian passive margin as right-lateral strike-slip faults in the Foothills during the Plio-Quaternary collision in Taiwan. We conclude that multisource and multiscale geomorphic studies combined with tectonic analysis in the field yield a significant contribution to the understanding of the structural and kinematic development of the Western Foothills at the front of the Taiwan collision belt.

Keywords: neotectonics; transfer fault zone; fold-and-thrust belt; Digital Elevation Model; drainage network; SPOT-E; SLAR; palaeostress; Western Foothills, Taiwan

1. Introduction Taiwan is located along a segment of the convergent boundary between the Philippine Sea and * Corresponding author. E-mail: [email protected]

Eurasian plates (e.g., Suppe, 1981; Barrier, 1985; Ho, 1986a,b, 1988; Angelier et al., 1986, 1990; Teng, 1990) (Fig. 1). Northeast of Taiwan, the Philippine Sea plate is subducting to the northwest beneath the Eurasian continental margin, whereas south of Taiwan the South China Sea floor, belonging to the

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B. Dep]bntaines et al./Tectonophvsics 274 (I 997) 61-82

Fig. 1. Geodynamic framework and main structural traits of Taiwan. White pattern: Coastal Plain. The frame indicates the investigated area, i.e.. the Western Foothills and the Coastal Plain. Heavy fines indicate major thrusts, triangles on upthrown side. L.V.--- Longitudinal Valley.

Eurasia plate, is subducting to the southeast beneath the Philippine Sea plate. Between these major arc-trench systems (Ryukyu and Luzon-Manila, respectively), where active subduction occurs, oblique collision dominates across the Taiwan segment of the plate boundary (Suppe, 1981, 1984). The orogen developed in Taiwan during the late Cenozoic, mainly since 5 Ma (Ho, 1986a,b), is consequent to this active collision. In this paper, we study the westernmost units of the collision belt, which correspond to two main geological domains: (1) the Western Foothills, composed of non-metamorphic shallow-marine to shelf clastic sediments, Miocene to Pleistocene in age, affected by NW-vergent folds and thrusts, and (2) the Western Coastal Plain, which represents a foredeep basin filled by clastic sediments bounding the undeformed Chinese continental passive margin.

The Western Foothills of Taiwan are structurally and sedimentologically well known because of studies carried out by the Chinese Petroleum Corporation lhereafter CPC; e.g., Geological Maps of Taiwan: Miaoli, CPC (1974), Taichung, CPC (1982)]. The Foothills display active asymmetric tblds and low-angle thrust faults, which affect the late Cenozoic continental deposits of the Toukoshan formation (mainly composed of conglomerates and sandstones). The overall magnitude of the deformation decreases from east, where tight folds and steeply dipping thrusts are numerous, to west, where gentle and open folds dominate (Ho, 1986a,b). Fold axes trend mainly N20°E and have steep to overturned western flanks commonly cut by WNWvergent thrust faults. The two major ddcollement levels correspond to the Wuchihshan formation of the Late Oligocene and the Nanchuang formation of the Late Miocene (Suppe, 1976). Minor ones also exist within the Pliocene strata (Ho, 1979, 1986a,b). Some authors (Ho, 1979, 1986a, b; Pelletier and Stephan, 1986) have suggested the occurrence of oblique structures within the Taiwan Foothills. The development of strike-slip faults, oblique at large angle to the belt trend, often in conjugate sets, was observed but was restricted to some areas of the fold-and-thrust belt (Ho, 1988). These features correspond to the Alishan area and the Shantzechiao-Chingshuikeng area in northern Taiwan (Ho, 1967). Two sets of strike-slip fault zones were evidenced based on displacement of topographic and stratigraphic units: (1) WNW to E - W right-lateral strike-slip faults (Biq, 1989), and (2) NNE to N-S left lateral strike-slip faults (Ho, 1979, 1988; Yeh and Yang, 1992). In a previous paper (Deffontaines et al., 1994), a multisource geomorphic analysis at the scale of the whole orogen emphasized the overall presence of oblique features within the Taiwan Foothills, but the tectonic significance of these features was not discussed in detail. In this paper, we do not discuss this geomorphic analysis of Taiwan again; we rather locus on the front zone of the Ibld-and-thrust belt and we aim at presenting a new accurate mapping of the Foothills structures, based on subsurface data (e.g., Sun, 1963, 1964), new remote sensing images (SPOT Panchromatic), high-resolution digital eleva-

B. Deffontaines et al. / Tectonophysics 274 (1997) 61-82

tion models, SLAR photographs, aerial photographs and local additional field work, which confirms the presence of structures trending mainly N140°E, oblique to the fold-and-thrust belt. Our purpose is also to demonstrate that these Nl40°E-trending structures correspond to transfer faults, that is to strike-slip faults parallel to the thrust transport direction which separate two parts of a given thrust sheet, each of which may have different displacements and deformations (McClay, 1992). We finally aim at discussing the significance of these transfer faults in the framework of the westward propagation of thrust units onto the Chinese passive margin, in response to its ongoing collision with the Luzon arc. It is first necessary to focus on the structure of the Western Foothills and the Coastal Plain structures, in order to provide new evidence for major N140°E-trending structures oblique to the active deformation front. We aim at locating precisely these structures and at defining their mechanisms during the Quaternary, based on geomorphic as well as structural and (palaeo)stress data. There is no need to discuss again both the geodynamic context and the general structure of Taiwan, which are summarized in Fig. 1 and discussed in other papers of this special issue.

2. Insights from numerical geomorphic approach and remote sensing analysis 2.1. The Digital Elevation Model (DEM) and its derivatives

We obtained local digital elevation models by digitizing isocontours of topographic maps of the Western Foothills of Taiwan (Fig. 2A), and using stereoscopic aerial photographs or spatial images. The DEM ground resolution is 50 x 50 m. Minor artefacts appear on DEM, mainly because of both the limits of the splin bicubic interpolation in the areas, and local alteration of topographic data (large buildings). In order to recognize the major morphological features, we examined additional information obtained through the analysis of parameters derived by numerical means (such as slopes or hill-shading maps; see Deffontaines et al., 1994 for examples).

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Computer programs (Carvalho, 1995) provided numerical morpho-tectonic and graphic outputs, such as for different simulated hill-shading pictures. It gives a good bird's-eye view of the structure and reveals the major geomorphic discontinuities (Fig. 2B). To analyze and interpret these documents (Fig. 2B), we used a lineament analysis based on structure, texture, shape, and colour (intensity hue and saturation: O'Leary et al., 1976; Scanvic, 1983), which correspond to qualitative interpretation criteria in remote sensing. Especially, alignments characterized by similar pixel values (colours) indicate lithological or structural linear features. In the DEM-derived maps, linear features striking obliquely or perpendicularly to the major structural trends were recognized, with smaller length and lesser continuity than for the well known major features (Fig. 2B). For instance, linear features trending NW-SE are clearly visible in the southern segments of the Pakuashan anticline (Fig. 2A, B). Where lithological contrasts are absent and thus cannot account for differential erosion, which is the most common case, these oblique and transverse features are tectonically significant. In contrast to longitudinal structures, many of these oblique features were not shown in the geological map of Taiwan (Ho, 1986a,b) and they are even absent in more detailed local maps. Furthermore, local geological studies in the field resulted in identification of major and minor fracture sets along such oblique trends, at various sites in the Western Foothills. The identification of strike-slip movements along the major fault lines, and in several cases the identification of linear features themselves as major fractures, are generally impossible to obtain from the DEM analysis solely. We point out, however, that despite this lack of direct evidence their existence is ascertained by the association of morphological studies and local analyses of fracture and fault patterns. 2.2. Drainage pattern and anomalies analysis

The Western Foothills drainage pattern (Fig. 2C) was simply extracted manually from topographic maps of scale l:100,000. In these maps, the drainage pattern is accurately described where all streams, including small dry talwegs, could be mapped. The regional drainage pattern of the stud-

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